A herd of adult and baby elephants walks in the dawn light as the highest mountain in Africa, Mt. Kilimanjaro in Tanzania, is seen in the background, in Amboseli National Park, southern Kenya, in December.

Ben Curtis/AP/File

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Nairobi, Kenya

Kenya has suffered its worst loss of elephants to poachers on record, with 12 members of one family slaughtered and their tusks hacked out in just a few hours last weekend.

Eleven adults and one infant calf died in a “targeted and efficient” attack highlighting the growing professionalism of poachers bankrolled by international criminals supplying soaring demand for ivory in the Far East. The calf, less than a year old, is believed to have been crushed by its dying mother as she fell to the ground.

“It is unimaginable, a heinous, heinous crime,” said Paul Udoto, spokesman for the Kenya Wildlife Service (KWS). “We have not seen such an incident in recent memory, it’s the worst single loss that we have on record, and our records go back almost 30 years. These were professional killers. The attack was targeted and efficient.”

The poachers, armed with automatic rifles, had already fled, but there were hopes Tuesday that a massive search involving foot patrols, a dozen vehicles, and three aircraft could still find them.

“Every possible resource is being deployed to track down these criminals,” Mr. Udoto said. “They will feel the full force of the law.”

But privately, conservations fear the poachers and their haul of 22 tusks, worth an estimated $281,000 on the Asian market, would already have escaped following the attack, which occurred late Saturday in a remote corner of Tsavo East National Park, Kenya’s largest wildlife reserve.

This was the latest in a surge of elephant deaths; the number of the animals killed for their ivory in Kenya increased sevenfold in five years, from fewer than 50 in 2007 to 360 in 2012, according to KWS figures. Over the past six weeks, 20 elephants have been found dead, with their tusks hacked out, in the Samburu ecosystem of northern Kenya alone. Three females were killed close to the Amboseli National Park in October. Experts speculate that many more are killed in the wilderness and their carcasses never found.

The increase has led many wildlife experts to declare the current situation a crisis worse even than the mass slaughter of Africa's elephants in the 1970s and '80s, which led to a global ivory trade ban in 1989.

“Now the situation is far graver, because we have fewer elephants left, but the demand for ivory is far greater," says Iain Douglas-Hamilton, founder of Save The Elephants. “The only thing that will radically alter the situation now is somehow to lower that demand.”

Two average 10 lb. tusks from an adult female elephant are now worth more than $20,000 in China, close to double their value a decade ago. The new demand is driven by the country’s booming middle class, for whom carved ivory and tusk trinkets are a sign of wealth.

Occasional “one-off sales” to China and Japan of stockpiled ivory from Southern Africa, most recently in 2008, are also blamed for restarting a market that had been dormant since the trade was banned.

"Africa has a half-million elephants left, but all together we know they are not enough to satisfy the demand for their ivory," adds Udoto, of the KWS. "We must all pull in one direction to stop that demand."